1. Field of the Invention
This invention relates to high voltage lithium ion battery components and more particularly this invention relates to high voltage reduction-oxidation shuttles and methods for their production.
2. Background of the Invention
Lithium ion batteries provide the best option to date for portable electric power. Their safety compared to early lithium batteries is multi-fold, and include the minimization of lithium metal within the battery. Whereas solid lithium comprised the anode (negative electrode) of the first lithium batteries, the anodes of lithium ion batteries comprise graphite or other intercalating media adapted to receive lithium cations.
Unlike lithium metal, which is still used in primary batteries, lithium ion (Li+) is very stable and unreactive. When intercalated in the negative electrode, its potential is much lower than when in the positive electrode (this difference of potential is the source of energy in every battery) but explosive reactivity is eliminated. The battery works with lithium ions shuttling from one electrode to the other through an electrolyte solution. They move spontaneously from the negative to the positive electrode during discharge giving up the energy stored. During the recharge process energy is spent relocating those ions back in the negative electrode.
During discharge, lithium ions spontaneously shuttle from the negative insertion electrode into the electrolyte and from the electrolyte into the positive insertion electrode. The electrolyte allows the diffusion of ions but prevents electrons flow. At the same time electrons spontaneously flow from the negative to the positive electrode: through the load. As discharge proceeds the potential (E) of each electrode shifts resulting in a decreasing difference between them and thus to a decreasing voltage as charge (Q) exits the battery.
During charge, lithium ions are forced out of the positive into the electrolyte and into the negative electrode. Electrons are injected into the negative and taken from the positive electrode. In doing the negative potential becomes more negative and the positive more positive, thus increasing the difference of potential which can be equated to the voltage.
Invariably, more energy goes into charging than what is provided in discharge. A goal of the invention is minimizing that difference.
Lithium ion batteries provide superior reversibility during charge/discharge cycles; longer battery lives result. However, care must be taken to prevent overcharging, as battery components (e.g., the positive electrode or cathode), can be otherwise damaged. This is becoming more relevant as newer high voltage cathode materials start to emerge. For example, while voltage limits for initial Li-ion batteries rarely exceeded 3.6 V, it is not unusual to see voltages in the range of 4.2 V and 4.5 V now being required in the automotive and aviation industries. About 3.6 V is nominal for commercial Li-ion batteries, 4.2 V is the highest seen commercially, and 4.5 V is seen in experimental, not yet commercialized cathode materials.
Attempts to prevent overcharge include the use of Battery Management Systems (BMS) and similar electronic devices. BMS comprises a circuit board that protects a cell from cell overvoltage and under voltage as well as current discharge and charge. If a short occurs in the wiring between the cells and the BMS, a thermal runaway could result from overcharging or from external excessive loading, as has been postulated in recent airliner electrical system malfunctions.
In light of the foregoing, nonmechanical systems for preventing battery malfunctions have been investigated. For example, chemical additives known as reduction-oxidation (redox) shuttles are often mixed in with battery electrolyte to absorb excess charge and protect the electrodes during recharge. The intention is for these redox moieties to “shuttle” the current and prevent the voltage applied during recharge from increasing past the potential of the shuttle. To date, there are no viable high voltage shuttles.
However, state of the art processes for producing redox-shuttles for lithium ion batteries require expensive starting materials, which are in many cases hazardous. For example, a current shuttle production protocol utilizes chloro-diisopropyl phosphine and 1,4-dibromo-2,5-dimethoxybenzene. Also, cryogenic conditions (−78 C) are required to minimize side reactions, which would otherwise occur as the current production protocols are very exothermic. As a consequence of these drawbacks, the costs for producing more than small amounts of the shuttles is prohibitive. Therefore, industrial scale up has been hampered.
A need exists in the art for an economic method for producing redox shuttles that emphasizes scalability for use in lithium-ion batteries. The method should not require expensive or hazardous chemicals or generate extensive secondary waste streams. The method should produce redox shuttles with enhanced solubilities compared to state of the art shuttles, so as to confer greater overcharge protection at higher voltages and higher current rate.